= Composite tare mass
{title2=$m_{\mathrm t}=\sum_i m_{\mathrm t,i}$}
A <composite tare mass> is the combined <tare mass> of several disjoint container or support components included within the chosen weighing boundary. If component $i$ contributes $m_{\mathrm t,i}$,
$$
m_{\mathrm t}=\sum_i m_{\mathrm t,i},\qquad m_{\mathrm n}=m_{\mathrm g}-m_{\mathrm t}.
$$
The boundary matters: include each unwanted load component once, and exclude everything that is not on the <load receptor>. OIML's tare and gross/net terminology supplies the underlying weighing distinction; the component sum here is an explicit additive <mass> model.<Non-automatic weighing instruments, Part 1>[T.5.2.1–T.5.2.3, printed page16]{ref}
For an original assembly, three empty crucibles each have mass $80\,\mathrm g$, their three lids each have mass $10\,\mathrm g$, and their common tray has mass $100\,\mathrm g$. The <composite tare mass> is
$$
m_{\mathrm t}=3(80+10)+100=370\,\mathrm g.
$$
If the loaded assembly has <gross mass> $970\,\mathrm g$, its material has <net mass> $600\,\mathrm g$. Subtracting the lids again after subtracting the complete $370\,\mathrm g$ tare would incorrectly give $570\,\mathrm g$.
One can weigh the empty assembled system directly or sum separately measured component masses. For independent component <standard uncertainties> of $0.10\,\mathrm g$ per crucible, $0.02\,\mathrm g$ per lid and $0.20\,\mathrm g$ for the tray, the sum has
$$
u(m_{\mathrm t})=\sqrt{3(0.10)^2+3(0.02)^2+(0.20)^2}\,\mathrm g\simeq0.267\,\mathrm g.
$$
Shared calibration errors require <covariance> terms instead of this independent-input calculation.<Guidelines for Evaluating and Expressing the Uncertainty of NIST Measurement Results by Barry N. Taylor and Chris E. Kuyatt>[AppendixA, equationA-3]{ref} A changed lid, retained material or substituted tray changes the assembly and can invalidate a previously stored tare.
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